Machines may include one or more hydraulic circuits for operation of the machine, including a hydraulic transmission for providing propulsion. The hydraulic transmission may include one or more hydraulic circuits including a hydraulic pump and a hydraulic motor. For example, some machines may include a variable displacement hydraulic pump and/or a variable displacement hydraulic motor. The hydraulic pump may be driven by a prime mover, and by varying the displacement and/or speed of the hydraulic pump, the amount of fluid pumped to the hydraulic motor may be controlled. Fluid pumped to the hydraulic motor causes it to drive an output shaft coupled to one or more traction devices, such as wheels and/or ground engaging tracks. By varying the ratio of the displacement of the hydraulic pump to the displacement of the hydraulic motor, the speed at which the hydraulic motor drives the output shaft may be controlled.
Under certain operational conditions, pressure may build in the hydraulic circuit to a level higher than desired, and thus, it may be desirable to regulate the pressure. For example, when movement of the machine is inhibited by an external resistance, pressure may build within the hydraulic circuit that drives the traction devices, for example, when the machine pushes against a large pile of earth. This may result in travel of the machine being slowed or stopped. The slowing or stopping of the machine may cause the traction devices to also be slowed or stopped, which, in turn, slows or stops the hydraulic motor coupled to the traction devices. As the hydraulic motor is slowed or stopped, the flow of fluid through the hydraulic motor may be substantially inhibited, even though the flow of fluid supplied from the hydraulic pump may continue at substantially the same flow rate. As a result, pressure in the circuit may increase beyond desired levels, which may be related to machine design limits and/or the physical characteristics of the fluid in the hydraulic circuit. As a result, it may be desirable to regulate pressure in the hydraulic transmission to prevent the pressure from reaching an undesirably high level while still enabling the operator to control the machine in a responsive manner.
One attempt to control a machine having a hydrostatic transmission is described in U.S. Pat. No. 7,146,263 B2 to Guven et al. (“the '263 patent”). The '263 patent discloses a method for predictive load management. According to the '263 patent, a control system is operable to receive at least one input indicative of a load on a transmission and to identify a desired load of the transmission based on the at least one input. The control system is also operable to receive at least one input indicative of current power output of a power source. The control system of the '263 patent limits the desired transmission load applied to the driven member of the transmission based on the current power output of the power source to thereby prevent the power source from operating outside the desired operating range.
Although the method disclosed in the '263 patent may prevent the power source from operating outside of a desired operating range, the method may suffer from a number of possible drawbacks. For example, the method described in the '263 patent may suffer from inefficiencies. In addition, the method disclosed in the '263 patent may not adequately regulate pressure in a hydraulic transmission while still enabling the operator to control the machine in a responsive manner.
The systems and methods disclosed herein may be directed to mitigating or overcoming one or more of the possible drawbacks set forth above.